Optical scanning device

ABSTRACT

An optical scanning device ( 1 ) for scanning three information layer ( 2, 2′, 2″ ) with three respective radiation beams ( 4, 4′, 4″ ) having three respective wavelengths (λ 1 , λ 2 , λ 3 ) and polarizations (p1, p2, p3). The three wavelengths differ from each other. At least one of the three polarizations differs from the others. The device comprises a diffractive part ( 24 ) including a pattern of pattern elements which have one stepped profile for forming three diffracted beams ( 15, 15′, 15″ ) from the three radiation beams, the part comprising birefiringent material, sensitive to the three polarizations. The stepped profile is designed such that the heights (h j ) of the steps of a pattern element introduce phase changes that equal at least two different multiples of 2π for one (λ l ) of the three wavelengths and equal at least two different phase changes modulo 2π for one (λ 2 ) of the two other wavelengths.

The present invention relates to an optical scanning device for scanninga first information layer by means of a first radiation beam having afirst wavelength and a first polarization, a second information layer bymeans of a second radiation beam having a second wavelength and a secondpolarization, and a third information layer by means of a thirdradiation beam having a third wavelength and a third polarization,wherein said first, second and third wavelengths substantially differfrom each other and at least one of said first, second and thirdpolarizations differs from the others, the device comprising:

-   -   a radiation source for supplying said first, second and third        radiation beams consecutively or simultaneously,    -   an objective lens system for converging said first, second and        third radiation beams on the positions of said first, second and        third information layers, respectively, and    -   a diffractive part arranged in the optical path of said first,        second and third radiation beams, the part including a pattern        of pattern elements which have substantially one stepped profile        for forming a first diffracted radiation beam, a second        diffracted radiation beam and a third diffracted radiation beam        from said first, second and third radiation beams, respectively,        the part comprising birefringent material sensitive to said        first, second and third polarizations.

More specifically, but not exclusively, the invention relates to anoptical scanning device compatible with three different formats, such ascompact discs (CDs), conventional digital versatile discs (DVDs) alsocalled “Red DVD” and so-called next generation DVDs also called “BlueDVD”.

The present invention also relates to a diffractive part for use in anoptical device for scanning a first information layer by means of afirst radiation beam having a first wavelength and a first polarization,a second information layer by means of a second radiation beam having asecond wavelength and a second polarization, and a third informationlayer by means of a third radiation beam having a third wavelength and athird polarization, wherein said first, second and third wavelengthssubstantially differ from each other and at least one of said first,second and third polarizations differs from the others, the diffractivepart:

-   -   being arranged in the optical path of said first, second and        third radiation beams,    -   including a pattern of pattern elements which have substantially        one stepped profile for forming a first diffracted radiation        beam, a second diffracted radiation beam and a third diffracted        radiation beam from said first, second and third radiation        beams, respectively, and    -   comprising birefringent material sensitive to said first, second        and third polarizations.

“Scanning an information layer” refers to scanning by means of aradiation beam for reading information in the information layer(“reading mode”), writing information in the information layer (“writingmode”), and/or erasing information in the information layer (“erasemode”). “Information density” refers to the amount of stored informationper unit area of the information layer. It is determined by, inter alia,the size of the scanning spot formed by the scanning device on theinformation layer to be scanned. The information density may beincreased by decreasing the size of the scanning spot. Since the size ofthe spot depends, inter alia, on the wavelength λ and the numericalaperture NA of the radiation beam forming the spot, the size of thescanning spot can be decreased by increasing NA and/or by decreasing λ.

A “diffracted radiation bearn” consists of a plurality of radiationbeams having each a diffraction order “m”, i.e. the zeroth order (m=0),the +1^(st)-order (m=1), the +2^(nd)-order (m=2), etc., the−1^(st)-order (m=−1), the −2^(nd)-order (m=−2), etc. It is noted that aradiation beam of the zeroth order is considered, in the presentdescription, as a diffracted radiation beam.

It is desirable for an optical scanning device to be compatible withdifferent formats of optical record carriers, i.e. for scanning opticalrecord carriers of different formats by means of radiation beams havingdifferent wavelengths whilst using one objective lens system. Forexample, CDs are available, inter alia, as CD-A (CD-audio), CD-ROM(CD-read only memory) and CD-R (CD-recordable), and are designed to bescanned by means of a radiation beam having a wavelength (λ) of about780 nm. Red-DVDs, on the other hand, are designed to be scanned by meansof a radiation beam having a wavelength of about 660 nm, and Blue-DVDsare designed to be scanned by means of a radiation beam having awavelength of about 405 nm. Notably, a “Blue DVD”-format disc has agreater data storage capacity than a “Red DVD”-format disc—typically atleast a twofold increase in storage capacity can be obtained.

The diversity of these formats raises the following difficulties.Firstly, discs designed for being read out at a certain wavelength arenot always readable at another wavelength. An example is a “CD-R”-formatdisc in which special dyes had to be applied in the recording stack inorder to obtain a high modulation for λ=785 nm. At λ=60 nm, themodulation of the signal from the disc becomes so small due to thewavelength sensitivity of the dye that readout at this wavelength is notfeasible. Secondly, when a new optical scanning system with higherstorage capacities is introduced, it is important for the new opticalscanning device to be backward compatible, i.e. to be able to scanoptical record carriers having already existing formats. Thirdly, thereis a difference in thickness between two discs having different formatssuch that spherical aberration is generated in one case and not in theother case.

As a result from this plurality of formats, a problem is to design andto make an optical scanning device capable of generating predefinedwavefronts for the wavelength associated to each format.

It has already known, for example, in the Japanese Patent applicationJP-A-2001209966, to provide an optical scanning device for scanning a“Blue-DVD”-format disc, a “Red-DVD”-format disc and a CD-format disc bymeans of a first radiation beam, a second radiation beam and a thirdradiation beam, respectively. The first, second and third radiationbeams have a first wavelength λ₁, a second wavelength λ₂ and a thirdwavelength λ₃, respectively, as well as a first polarization p1, asecond polarization p2, and a third polarization p3, respectively. Thewavelengths λ₁, λ₂ and λ₃ differ from each other. At least one of thepolarizations p1, p2 and p3 differs from the others. Furthermore, theknown optical scanning device includes a radiation source for supplyingthe three radiation beams, an objective lens system for converging thethree radiation beams in information layers of the three optical recordcarriers, respectively, and a diffractive part arranged in the opticalpath of the three radiation beams. The objective lens system has anoptical axis. The diffractive part has two parallel planes between whicha first layer made of glass and a second layer are provided. Theinterface between the first and second layers is a pattern of patternelements having one stepped profile. The choice of the materials of thefirst and second layers and the design of the stepped profile are suchthat the diffractive part forms a first diffracted radiation beam of thezeroth order for the wavelength λ₁ and a diffracted radiation beam of ahigher (i.e. non-zeroth) order for each of the wavelengths λ₂ and λ₃.Furthermore, the second layer is made of a birefringent materialsensitive to the polarizations p1, p2 and p3.

Said Japanese Patent application JP-A-2001209966 teaches two solutionsfor making the second layer of the diffractive part.

In the first known solution, the second layer is made of a LiquidCrystal (LC) material that is electrically adjustable (by means ofelectrodes) for modifying its refractive index so as to form the threediffracted beams. As a result, such a device is complex to design andrequires the making of the switchable LC-component which is difficultand expensive to design and to make.

In the second known solution, the second layer is made of a solidbirefringent material having an ordinary index and an extraordinaryindex, one of which equals the refractive index of the first layer(glass). Thus, where the first polarization is aligned with thedirection associated with the latter index, the first diffracted beamemerging from the diffractive part has a flat wavefront: the zerothorder of the first diffracted beam is then formed. In other words, thediffractive part then acts as a transparent parallel plate for the firstwavelength.

Accordingly, it is an object of the present invention to provide anoptical scanning device suitable for scanning optical record carriers bymeans of radiation beams having three different wavelengths and havingdifferent polarizations, the device being an alternative to the knownsolutions.

This object is reached by an optical scanning device as described in theopening paragraph wherein, according to the invention, said steppedprofile is designed such that the heights of the steps of a patternelement introduce phase changes that substantially equal at least twodifferent multiples of 2π for said first wavelength and at least twosubstantially different phase changes modulo 2π for said secondwavelength. It will be noted that the values of the two substantiallydifferent phase changes in respect of said second wavelength can bechosen among a plurality of at least three values as described below infurther detail.

By contrast to the solutions known from said Japanese Patent applicationJP-A-2001209966, the diffractive part according to the invention formsthe zeroth order of the first diffracted beam as follows. Thus, wherethe first polarization is aligned with the direction of either theordinary axis or the extraordinary axis, phase changes thatsubstantially equal at least two different multiples of 2π areintroduced in the first diffracted beam, as a result of the design ofthe heights of the steps of a pattern element of the stepped profile.Accordingly, the zeroth order of the first diffracted beam is formed. Inother words, in the diffractive part according to the invention, unequaloptical path lengths are introduced in the first diffraction beam in adirection perpendicular to the optical axis of the objective lens(called in the following “radial direction”). It is noted that thisdiffractive part does not acts as a transparent parallel plate for thefirst wavelength, since the first diffracted beam does not have a flatwavefront, as opposed to the known diffractive part.

An advantage of the stepped profile according to the invention is toprovide a diffractive part that forms said first and second diffractedbeams with predetermined values, e.g. high values, of transmissionefficiencies for desired orders of these beams. Additionally, thediffractive part may form the second diffractive part with a phasechange that approximates the ideal sawtooth-like profile as describedbelow in further detail.

Notably, it is known from the European Patent Application filed on Apr.9, 2000 under the application number 00203066.6, to provide an opticalscanning device with a diffractive part that includes a pattern ofpattern elements having a stepped profile designed such that the opticalpaths pertaining to steps of said pattern element are substantiallyequal to multiples of said first wavelength. Such a diffractive part isadvantageous since it allows the formation of diffracted beams withselected diffraction orders (i.e. orders for which a high transmissionefficiency is achieved), which is otherwise difficult due to theplurality of wavelengths. However, the teaching of that Application doesnot provide sufficient guidance for providing the optical scanningdevice according to the invention.

Firstly, that European Patent Application does not teach how to designan optical scanning device that is compatible with three differentformats of optical record carriers, but only describes a device forscanning optical record carriers of two different formats by means oftwo radiation beams having two wavelengths.

Secondly, the European Patent Application does not teach an easy methodfor making a diffractive part with a stepped profile. The fixed valuesof the three wavelengths are a severe constraint when designing thediffractive part using the method explained in that Application. Morespecifically, when a radiation beam having a wavelength λ traverses astep made of a material having a step height h, a phase change Φ (withrespect to the case where the radiation beam traverses the air along thesame distance) is introduced in the diffracted beam emerging form thestep. The phase change Φ is given by the following equation:$\begin{matrix}{\Phi = {2\quad\pi\frac{h\left( {n - n_{0}} \right)}{\lambda}}} & (0)\end{matrix}$where “n” is the refractive index of the diffractive part and “n₀” isthe refractive index of the adjacent medium. It follows from Equation(0) that, when the wavelength λ changes, the phase change Φ changesaccordingly. Thus, designing a diffractive part compatible for operatingwith three wavelengths with a stepped profile would require to design avery complex stepped profile having relatively high steps in order tohave high efficiency for each of three wavelengths. This results in adiffractive part that is difficult to make.

Thirdly, that European Patent Application does not teach how to design apolarization-sensitive diffractive part such that the optical scanningdevice can operate with radiation beams having different polarizations.

It is also known from the Japanese Patent Application JP 2001-174614 tomake an optical scanning device comprising a diffractive part made ofbirefringent material, designed to be polarization-sensitive andwavelength-sensitive so as to form a first, zeroth-order radiation beamhaving a first wavelength and a first polarization, a second,non-zeroth-order radiation beam having the first wavelength and asecond, different polarization, and a third radiation beam having asecond, different wavelength and the first or second polarization.However, that Japanese Patent Application does not teach how to designthe diffractive part such that the optical scanning device can operatewith three different wavelengths.

It is also known from the Japanese Patent Application JP 2001-174614 tomake an optical scanning device comprising a diffractive part made ofbirefringent material, designed to be polarization-sensitive andwavelength-sensitive so as to form a first, zeroth-order radiation beamhaving a first wavelength and a first polarization, a second,non-zeroth-order radiation beam having the first wavelength and asecond, different polarization, and a third radiation beam having asecond, different wavelength and the first or second polarization.However, that Japanese Patent Application does not teach how to designthe diffractive part such that the optical scanning device can operatewith three different wavelengths.

It is also known from the Japanese Patent Application JP 2001-195769 tomake an optical scanning device suitable for scanning optical recordcarriers by means of three radiation beams having three differentwavelengths, the device comprising a diffractive part. However, thatJapanese Patent Application does not teach how to provide an opticalscanning device for scanning optical record carriers by means ofradiation beams having different polarizations. In particular, thatJapanese Patent Application does not describe or suggest how to make apolarization-sensitive diffractive part and, more specifically, it doesnot mention the use of birefringent material for making the diffractivepart.

Furthermore, it is known from numerous patent documents, e.g.JP-2001043559, to provide an optical scanning device comprising adiffractive part operating with two radiation beams having two differentwavelengths. However, none of these documents mention the use ofbirefringent material to make a polarization-sensitive diffractive partsuch that the optical scanning device can operate with radiation beamshaving different polarizations.

In a first embodiment of the optical scanning device according to thepresent invention, said stepped profile is further designed such thatthe heights of the steps of a pattern element introduce phase changesthat substantially equal at least two substantially different phasechanges modulo 2π for said third wavelength. Similarly to the secondwavelength, it is noted that the values of the two substantiallydifferent phase changes in respect of said third wavelength can bechosen among a plurality of at least three values. In a particular caseof this first embodiment, said stepped profile is further designed suchthat the heights of the steps of a pattern element introducesubstantially identical phase changes for both said second and thirdwavelengths, wherein said third polarization differs from said secondpolarization.

In a second embodiment of the optical scanning device according to thepresent invention, said stepped profile is designed such that theheights of the steps of a pattern element introduce phase changes thatsubstantially equal at least two different multiples of 2π for saidthird wavelength. In a particular case of this second embodiment, saidstepped profile is further designed such that the heights of the stepsof a pattern element introduce substantially identical phase changes forboth said first and third wavelengths, wherein said third polarizationdiffers from said first polarization.

In a third embodiment of the optical scanning device according to thepresent invention, said stepped profile is designed such that theheights of the steps of a pattern element introduce phase changes thatsubstantially equal at least two different odd multiples of n for saidthird wavelength. In a particular case of the third embodiment, saidstepped profile is further designed such that the heights of the stepsof a pattern element introduce phase changes that substantially equal atleast two substantially different phase changes for said secondwavelength. It will be noted that these two substantially differentphase changes for said second wavelength are chosen among an odd numberof substantially different phase changes.

It will be noted that if the first, second and third polarizations areidentical, only two different values (zero and π modulo 2π) can bechosen for the phase changes in respect of the second or thirddiffracted beam. Therefore, the stepped profile cannot be designed withpredetermined values, e.g. high values, of transmission efficiencies fordesired orders of each of the first, second and third diffracted beams.By contrast, it will be noted that if at least one of the first, secondand third polarizations differs from the others, at least threedifferent values can be chosen for each of the second and thirddiffracted beams, thereby resulting in allowing the design of thestepped profile with a relatively low number of steps, typically lessthan 40 steps, since a stepped profile with a high number of steps(typically, 50 or more steps) is of less practical use.

Another object of the present invention is to provide a diffractive partfor use in an optical device for scanning a first information layer bymeans of a first radiation beam having a first wavelength and a firstpolarization, a second information layer by means of a second radiationbeam having a second wavelength and a second polarization, and a thirdinformation layer by means of a third radiation beam having a thirdwavelength and a third polarization, wherein said first, second andthird wavelengths substantially differ from each other and at least oneof said first, second and third polarizations differs from the others,the diffractive part being an alternative to the known part.

This object is reached by a diffractive part as described in the openingparagraph wherein, according to the invention, characterized in thatsaid stepped profile is designed such that the heights of the steps of apattern element introduce phase changes that substantially equal atleast two different multiples of 2π for said first wavelength, at leasttwo substantially different phase changes modulo 2π for said secondwavelength and, for said third wavelength, one of the following: atleast two substantially different phase changes modulo 2π, at least twodifferent multiples of 2π, or at least two different odd multiples of π.

The objects, advantages and features of the invention will be apparentfrom the following, more detailed description of the invention, asillustrated in the accompanying drawings, in which:

FIG. 1 is a schematic illustration of components of an optical scanningdevice 1 according to the invention,

FIG. 2 is a schematic illustration of an objective lens for use in thescanning device of FIG. 1,

FIG. 3 is a schematic front view of the objective lens of FIG. 2,

FIG. 4 shows a curve representing a phase change introduced by thediffractive part shown in FIGS. 2 and 3, in the form of a sawtooth-likefunction (ideal case),

FIG. 5 shows a curve representing a phase change introduced by thediffractive part shown in FIGS. 2 and 3, in the form of a steppedfunction (approximation case),

FIG. 6 shows a curve representing the step heights of a first example ofa first embodiment of the diffractive part shown in FIGS. 2 and 3,

FIG. 7 shows a curve representing the step heights of a second exampleof a first embodiment of the diffractive part shown in FIGS. 2 and 3,

FIG. 8 shows a curve representing the step heights of a third example ofa first embodiment of the diffractive part shown in FIGS. 2 and 3,

FIG. 9 shows a curve representing the step heights of a secondembodiment of the diffractive part shown in FIGS. 2 and 3, and

FIG. 10 shows a curve representing the step heights of a thirdembodiment of the diffractive part shown in FIGS. 2 and 3.

FIG. 1 is a schematic illustration of the optical components of anoptical scanning device I according to one embodiment of the invention,for scanning a first information layer 2 of a first optical recordcarrier 3 by means of a first radiation beam 4.

By way of illustration, the optical record carrier 3 includes atransparent layer 5 on one side of which the information layer 2 isarranged. The side of the information layer facing away from thetransparent layer 5 is protected from environmental influences by aprotective layer 6. The transparent layer 5 acts as a substrate for theoptical record carrier 3 by providing mechanical support for theinformation layer 2. Alternatively, the transparent layer 5 may have thesole function of protecting the information layer 2, while themechanical support is provided by a layer on the other side of theinformation layer 2, for instance by the protective layer 6 or by anadditional information layer and transparent layer connected to theuppermost information layer. It is noted that the information layer hasa first information layer depth 27 that corresponds to, in thisembodiment as shown in FIG. 1, to the thickness of the transparent layer5. The information layer 2 is a surface of the carrier 3. That surfacecontains at least one track, i.e. a path to be followed by the spot of afocused radiation on which path optically-readable marks are arranged torepresent information. The marks may be, e.g., in the form of pits orareas with a reflection coefficient or a direction of magnetizationdifferent from the surroundings. In the case where the optical recordcarrier 3 has the shape of a disc, the following is defined with respectto a given track: the “radial direction” is the direction of a referenceaxis, the X-axis, between the track and the center of the disc and the“tangential direction” is the direction of another axis, the Y-axis,that is tangential to the track and perpendicular to the X-axis.

As shown in FIG. 1, the optical scanning device 1 includes a radiationsource 7, a collimator lens 18, a beam splitter 9, an objective lenssystem 8 having an optical axis 19, a diffractive part 24, and adetection system 10. Furthermore, the optical scanning device 1 includesa servocircuit 11, a focus actuator 12, a radial actuator 13, and aninformation processing unit 14 for error correction.

In the following “Z-axis” corresponds to the optical axis 19 of theobjective lens system 8. It is noted that (X, Y, Z) is an orthogonalbase.

The radiation source 7 is arranged for consecutively or simultaneouslysupplying the radiation beam 4, a second radiation 4′ (not shown inFIG. 1) and a third radiation beam 4″ (not shown in FIG. 1). Forexample, the radiation source 7 may comprise either a tunablesemiconductor laser for consecutively supplying the radiation beams 4,4′ and 4″ or three semiconductor lasers for simultaneously supplyingthese radiation beams. Furthermore, the radiation beam 4 has awavelength λ₁ and a polarization p₁, the radiation beam 4′ has awavelength λ₂ and a polarization p₂, and the radiation beam 4″ has awavelength λ₃ and a polarization p₃. The wavelengths λ₁, λ₂ and λ₃differ substantially from each other and at least two of thepolarizations p₁, p₂ and p₃ differ from each other. Examples of thewavelengths λ₁, λ₂ and λ₃ and the polarizations p₁, p₂ and p₃ will begiven below.

The collimator lens 18 is arranged on the optical axis 19 fortransforming the radiation beam 4 into a substantially collimated beam20. Similarly, it transforms the radiation beams 4′ and 4″ into tworespective substantially collimated beams 20′ and 20″ (not shown in FIG.1).

The beam splitter 9 is arranged for transmitting the collimatedradiation beam toward the objective lens system 8. Preferably, the beamsplitter 9 is formed with a plane parallel plate that is tilted with anangle α with respect to the Z-axis and, more preferably, α=45°.

The objective lens system 8 is arranged for transforming the collimatedradiation beam 20 to a first focused radiation beam 15 so as to form afirst scanning spot 16 in the position of the information layer 2. Inthis embodiment, the objective lens system 8 includes an objective lens17 provided with the diffractive part 24.

The diffractive part 24 includes birefringent material having anextraordinary refractive index ne and an ordinary refractive index no.In the following the change in refractive index due to difference inwavelength is neglected and therefore the refractive indices ne and noare approximately independent of the wavelength. In this embodiment, andby way of illustration only, the birefringent material is C6M/E7 50/50(in % by weight) with n₀=1.51 and n_(e)=1.70. Alternatively, forexample, the birefringent material may be C6M/C3M/E7 40/10/50 (in % byweight) with n₀=1.55 and n_(e)=1.69. The codes used refer to thefollowing substances:

-   E7: 51% C5H11 cyanobiphenyl, 25% C5H15cyanobiphenyl, 16%    C8H17cyanobiphenyl, 8% C5H₁₁ cyanotriphenyl;-   C3M: 4-(6-acryloyloxypropyloxy) benzoyloxy-2-methylphenyl    4-(6-acryloyloxypropyloxy) benzoate;-   C6M: 4-(6-acryloyloxyhexyloxy)benzoyloxy-2-methylphenyl    4-(6-acryloyloxyhexyloxy) benzoate.

The diffractive part 24 is aligned such that the optic axis of thebirefringent material is along the Z-axis. It is also aligned such thatits refractive index equals ne when traversed by a radiation beam havinga polarization along the X-axis and n₀ when traversed by a radiationbeam having a polarization along the Y-axis. In the following thepolarization of a radiation beam is called “p_(e)” and “p_(o)” wherealigned with the X-axis and the Y-axis, respectively. Thus, where thepolarization p₁, p₂ or p₃ equals p_(e), the refractive index of thebirefringent material equals n_(e) and, where the polarization p₁, p₂ orp₃ equals p_(o), the refractive index of the birefringent materialequals n_(o). In other words, the birefringent diffractive part 24 soaligned is sensitive to the polarizations p₁, p₂ and p₃. The diffractivepart 24 will be described in further detail.

During scanning, the record carrier 3 rotates on a spindle (not shown inFIG. 1) and the information layer 2 is then scanned through thetransparent layer 5. The focused radiation beam 15 reflects on theinformation layer 2, thereby forming a reflected beam 21 which returnson the optical path of the forward converging beam 15. The objectivelens system 8 transforms the reflected radiation beam 21 to a reflectedcollimated radiation beam 22. The beam splitter 9 separates the forwardradiation beam 20 from the reflected radiation beam 22 by transmittingat least a part of the reflected radiation beam 22 towards the detectionsystem 10.

The detection system 6 includes a convergent lens 25 and a quadrantdetector 23 which are arranged for capturing said part of the reflectedradiation beam 22 and converting it to one or more electrical signals.One of the signals is an information signal I_(data), the value of whichrepresents the information scanned on the information layer 2. Theinformation signal I_(data) is processed by the information processingunit 14 for error correction. Other signals from the detection system 10are a focus error signal I_(focus) and a radial tracking error signalI_(radial). The signal I_(focus) represents the axial difference inheight along the Z-axis between the scanning spot 16 and the position ofthe information layer 2. Preferably, this signal is formed by the“astigmatic method” which is known from, inter alia, the book by G.Bouwhuis, J. Braat, A. Huijser et al, entitled “Principles of OpticalDisc Systems,” pp.75-80 (Adam Hilger 1985) (ISBN 0-85274-785-3). Theradial tracking error signal I_(radial) represents the distance in theXY-plane of the information layer 2 between the scanning spot 16 and thecenter of a track in the information layer 2 to be followed by thescanning spot 16. Preferably, this signal is formed from the “radialpush-pull method” which is known from, inter alia, the book by G.Bouwhuis, pp.70-73.

The servocircuit 11 is arranged for, in response to the signalsI_(focus) and I_(radial), providing servo control signals I_(control)for controlling the focus actuator 12 and the radial actuator 13,respectively. The focus actuator 12 controls the position of theobjective lens 17 along the Z-axis, thereby controlling the position ofthe scanning spot 16 such that it coincides substantially with the planeof the information layer 2. The radial actuator 13 controls the positionof the objective lens 17 along the X-axis, thereby controlling theradial position of the scanning spot 16 such that it coincidessubstantially with the center line of the track to be followed in theinformation layer 2.

FIG. 2 is a schematic illustration of the objective lens 17 for use inthe scanning device 1 described above.

The objective lens 17 is arranged for transforming the collimatedradiation beam 20 to the focused radiation beam 15, having a firstnumerical aperture NA₁, so as to form the scanning spot 16. In otherwords, the optical scanning device 1 is capable of scanning the firstinformation layer 2 by means of the radiation beam 15 having thewavelength λ₁, the polarization p₁ and the numerical aperture NA₁.

Furthermore, the optical scanning device 1 is also capable of scanning asecond information layer 2′ of a second optical record carrier 3′ bymeans of the radiation beam 4′ and a third information layer 2″ of athird optical record carrier 3″ by means of the radiation beam 4″. Thus,the objective lens 17 transforms the collimated radiation beam 20′ to asecond focused radiation beam 15′, having a second numerical apertureNA₂, so as to form a second scanning spot 16′ in the position of theinformation layer 2′. The objective lens 17 also transforms thecollimated radiation beam 20″ to a third focused radiation beam 15″,having a third numerical aperture NA₃, so as to form a third scanningspot 16″ in the position of the information layer 2″.

Similarly to the optical record carrier 3, the optical record carrier 3′includes a second transparent layer 5′ on one side of which theinformation layer 2′ is arranged with a second information layer depth27′, and the optical record carrier 3″ includes a third transparentlayer 5″ on one side of which the information layer 2″ is arranged witha third information layer depth 27″. In this embodiment, the opticalrecord carriers 3, 3′ and 3″ are, by way of example only, a“Blue-DVD”-format disc, a “Red-DVD”-format disc and a CD-format disc,respectively. Thus, the wavelength λ₁ is comprised in the range between365 and 445 nm and, preferably, 405 nm. The numerical aperture NA₁equals about 0.6 in the reading mode and is above 0.6, preferably 0.65,in the writing mode. The wavelength λ₂ is comprised in the range between620 and 700 nm and, preferably, 660 nm. The numerical aperture NA₂equals about 0.6 in the reading mode and is above 0.6, preferably 0.65,in the writing mode. The wavelength λ₃ is comprised in the range between740 and 820 nm and, preferably, 785 nm. The numerical aperture NA₃ isbelow 0.5, preferably 0.45.

It is noted in the present description that two wavelengths λ_(a) andλ_(b) are substantially different from each other where |λ_(a)−λ_(b)| isequal to or higher than, preferably, 20 nm and, more preferably, 50 nm,where the values 20 and 50 nm are a matter of a purely arbitrary choice.

It is also noted that scanning information layers of the record carriers3, 3′ and 3″ of different formats is achieved by forming the objectivelens 17 as a hybrid lens, i.e. a lens combining diffractive andrefractive elements, used in an infinite-conjugate mode. Such a hybridlens can be formed by applying a grating profile on the entrance surfaceof the lens 17, for example by a lithographic process using thephotopolymerization of, e.g., an UV curing lacquer, therebyadvantageously resulting in the diffractive part 24 to be easy to make.Alternatively, such a hybrid lens can be made by diamond turning.

In this embodiment shown in FIGS. 1 and 2, the objective lens 17 isformed as a convex-convex lens; however, other lens element types suchas plano-convex or convex-concave lenses can be used. In thisembodiment, the diffractive part 24 is arranged on the side of a firstobjective lens 17 facing the radiation source 7 (referred to herein asthe “entrance face”).

Alternatively, the diffractive part 24 is arranged on the other surfaceof the lens 17 (referred to herein as the “exit face”). Alsoalternatively, the objective lens 10 is, for example, a refractiveobjective lens element provided with a planar lens diffractive elementforming the diffractive part 24. Also alternatively, the diffractivepart 24 is provided on an optical element separate from the objectivelens system 8, for example on a beam splitter or a quarter wavelengthplate.

Also alternatively, whilst the objective lens 10 is in this embodiment asingle lens, it may be a compound lens containing two or more lenselement. For instance, the objective lens system 8 may include anadditional objective lens that forms a doublet-lens system incooperation with the objective lens 17. The additional objective lensmay be plano-convex with a convex surface facing the objective lens 17and a flat surface facing the position of the information layer 2. Thisdoublet-lens system has advantageously a larger tolerance in mutualposition of the optical elements than the single-lens system. In thecase where NA>0.45, the additional objective lens is preferably formedby an aspherical lens.

FIG. 3 is a schematic view of the entrance surface (also called “frontview”) of the objective lens 17 shown in FIG. 2, illustrating thediffractive part 24.

The diffractive part 24 includes a pattern of pattern elements alsocalled “zones”. Each zone has a stepped profile. The stepped profiles ofthe zones are substantially identical. That stepped profile includes aplurality of “subzones” or “steps” having each a step height. In thefollowing the stepped profile is designed for introducing apredetermined phase change Φ which approximates a sawtooth-likefunction. FIG. 4 shows a curve 78 representing the phase change Φ in theform of a sawtooth-like function (which is the ideal case). It is notedthat FIG. 4 shows only the phase change Φ introduced by one patternelement of the diffractive part 24, where the phase change (is a linearfunction of “x”, the coordinate along the X-axis (in the radialdirection). It is known, e.g. from said European Patent applicationfiled under the application number 00203066.6, that the sawtooth-likefunction of the phase change Φ may be approximated by the followingstepped function: $\begin{matrix}{{\Phi(x)} = {{2\quad\pi\frac{{2j} - 1}{2P}\quad{for}\quad\frac{j - 1}{P}} \leq x \leq \frac{j}{P}}} & (1)\end{matrix}$where “P” is an integer representing the number of steps or “subzones”,“j” is an integer comprised between 1 and P which represents the stepnumber of each step.

FIG. 5 shows a curve 79 representing the phase change Φ introduced bythe diffractive part 24 in the approximation form of a stepped function.It is noted that FIG. 5 shows only the phase change Φ introduced by onepattern element of the diffractive part 24.

In the following “h” is the step height of a step of the steppedprofile, which is a function dependent on x. In the case of theapproximation of the phase change Φ according to Equation (1), the stepheight h is given by the following function: $\begin{matrix}{{h(x)} = {{h_{j}\quad{for}\quad\frac{j - 1}{P}} \leq x \leq \frac{j}{P}}} & \left( {2a} \right)\end{matrix}$where “h_(j)” is the step height of the step j, which is a constantparameter.

When designing the stepped profile of the diffractive part 24, the stepheight h are chosen so that the stepped profile introduces predeterminedvalues of the phase change Φ depending on the wavelength λ and thepolarization p of the diffracted beam emerging from the diffractive part24. Thus, in the following the phase change Φ is also noted Φ(λ, p) andthe step height h_(j) are chosen so that the stepped profile introducesa first value Φ(λ=λ₁, p=p₁) for the diffracted beam 15, a second valueΦ(λ=λ₂, p=p₂) for the diffracted beam 15′, and a third value Φ(λ=λ₃,p=p₃) for the diffracted beam 15″.

In the following and with reference to said European Patent applicationfiled under the application number 00203066.6, the wavelength λ₁ ischosen to be the design wavelength λ_(ref). In other words,λ_(ref)=λ₁  (2b)

Accordingly, the stepped profile is designed such that the phase changeΦ(λ=λ₁, p=p₁) for the radiation beam 15 substantially equals a multipleof 2λ, i.e. substantially equal zero modulo 2π. Thus, Φ(λ=λ₁, p=p₁)≡0(2π).

This is achieved when each step height h_(j) is a multiple of areference height h_(ref) which is dependent on the design wavelengthλ_(ref) as follows: $\begin{matrix}{{h_{ref}\left( \lambda_{ref} \right)} = \frac{\lambda_{ref}}{n - n_{0}}} & (3)\end{matrix}$where “n” is the refractive index of the diffractive part 24 and n₀ isthe refractive index of the adjacent medium that is, in the followingand by way of illustration only, air, i.e. n₀=1.

Since the diffractive part 24 is made of birefringent material, itsrefractive index n equals n_(e) when the polarization of the radiationbeam traversing the diffractive part 24 equals p_(e) and equals n_(o)when the polarization of the radiation beam traversing the diffractivepart 24 equals p_(o). Consequently, the reference height h_(ref) is alsodependent on the polarization p. Thus, in the following the phase changeh_(ref) is also noted h_(ref)(λ, p) and it follows from Equations (2b)and (3) that: $\begin{matrix}{{h_{ref}\left( {{\lambda = \lambda_{1}},{p_{1} = p_{e}}} \right)} = \frac{\lambda_{1}}{n_{e} - n_{0}}} & \left( {4a} \right) \\{{h_{ref}\left( {{\lambda = \lambda_{1}},{p_{1} = p_{o}}} \right)} = \frac{\lambda_{1}}{n_{o} - n_{0}}} & \left( {4b} \right)\end{matrix}$Accordingly, in the case where, e.g., n_(o)=1.50, n_(e)=1.60 and λ₁=405nm, the following is obtained from Equations (4a) and (4b):h _(ref)(λ=λ₁ , p ₁ =p _(o))=0.810 μm andh _(ref)(λ=λ₁ , p ₁ =p _(e))=0.675 μm.It is also noted that, while a step height h_(j) introduces the valueΦ(λ=λ₁, p=p₁) (substantially equal to zero modulo 2π) for the radiationbeam 15, it introduces the values Φ(λ=λ₂, p=p₂) and φ(λ=λ₃, p=p₃) forthe radiation beams 15′ and 15″, respectively, as follows:$\begin{matrix}{{\Phi\left( {{\lambda = \lambda_{2}},{p_{2} = p_{e}}} \right)} = {2\quad\pi\frac{n_{e} - n_{0}}{\lambda_{2}}{h_{ref}\left( {{\lambda_{ref} = \lambda_{1}},{p = p_{1}}} \right)}}} & \left( {5a} \right) \\{{\Phi\left( {{\lambda = \lambda_{2}},{p_{2} = p_{o}}} \right)} = {2\quad\pi\frac{n_{o} - n_{0}}{\lambda_{2}}{h_{ref}\left( {{\lambda_{ref} = \lambda_{1}},{p = p_{1}}} \right)}}} & \left( {5b} \right) \\{{\Phi\left( {{\lambda = \lambda_{3}},{p_{3} = p_{e}}} \right)} = {2\quad\pi\frac{n_{e} - n_{0}}{\lambda_{3}}{h_{ref}\left( {{\lambda_{ref} = \lambda_{1}},{p = p_{1}}} \right)}}} & \left( {5c} \right) \\{{\Phi\left( {{\lambda = \lambda_{3}},{p_{3} = p_{o}}} \right)} = {2\quad\pi\frac{n_{o} - n_{0}}{\lambda_{3}}{h_{ref}\left( {{\lambda_{ref} = \lambda_{1}},{p = p_{1}}} \right)}}} & \left( {5d} \right)\end{matrix}$

Table I shows the ideal values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) where theradiation beams 15′ and 15″ traverse the step height h_(j) which equalseither h_(ref)(λ_(ref)=λ₁, p₁=p_(e)) or h_(ref)(λ_(ref)=80 ₁, p₁=p_(o)),in the cases where the polarizations p₂ and p₃ equal p_(e) and/or p_(o).The values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) have been calculated fromEquations (4a), (4b) and (5a) to (5d) with, e.g., n_(o)=1.50, 4=1.60,λ₁=405 nm, λ₂=650 nm and λ₃=785 nm. TABLE I Φ(λ = λ₂, Φ(λ = λ₃, p =p₂)/2π p = p₃)/2π (modulo 1) (modulo 1) p₂ = p_(e) p₂ = p_(o) p₃ = p_(e)p₃ = p_(o) h_(j) = h_(ref)(λ_(ref) = λ₁, p = p₁) p₁ = p_(e) 0.623 0.5190.516 0.430 p₁ = p_(o) 0.748 0.623 0.619 0.516

It is further noted that a step height h_(j) equal to a multiple ofh_(ref)(λ=λ₁p=p₁) introduces the value Φ(λ=λ₁, p=p₁) that equals zeromodulo 2π for the diffracted beam 15 and the values Φ(λ=λ₂, p=p₂) andΦ(λ₃, p=p₃) that each equal one among a limited number of possiblevalues. In the following “#Φ(λ=λ₂, p=p₂)” and “#Φ(λ=λ₃, p=p₃)” are suchlimited numbers for the values Φ(λ=₂, p=p₂) and Φ(λ=λ₃, p=p₃),respectively. The limited numbers #Φ(λ=λ₂, p=p₂) and #Φ(λ=λ₃, p=p₃) havebeen calculated based on the theory of Continued Fractions, as knownfrom, e.g., the European patent application filed on 05.04.2001 underthe application number 01201255.5.

By way of illustration only, the calculation of the limited numbers#Φ(λ=λ₃, p=p₃) is now described in a first case where the polarizationsp₁ and p₃ are identical, e.g. p₁=p_(e) and p₃=p_(e), and a second casewhere the polarization p₁ differs from the polarization p₃, e.g.p₁=p_(e) and p₃=p_(o). With reference to said European patentapplication filed under the application number 01201255.5, the followingis defined: $\begin{matrix}{a_{0} = \frac{H_{1}}{H_{i}}} & \left( {6a} \right) \\{b_{0} = {{Int}\left\lbrack a_{o} \right\rbrack}} & \left( {6b} \right) \\{a_{1} = {a_{0} - b_{0}}} & \left( {6c} \right) \\{b_{m} = {{Int}\left\lbrack \frac{1}{a_{m}} \right\rbrack}} & \left( {6d} \right) \\{a_{m + 1} = {\frac{1}{a_{m}} - b_{m}}} & \left( {6e} \right) \\{{CF}_{m} \equiv \left\{ {b_{0},{b_{1}\quad\ldots\quad b_{m}}} \right\}} & \left( {6f} \right)\end{matrix}$where H₁=h_(ref)(λ=λ₁, p=p₁), H₁=h_(ref)(λ=λ₃, p=p₃) and “m” is aninteger equal to or higher than 1.

In the first case where p₁=p_(e) and p₃=p_(e) and where, e.g.,n_(o)=1.50, n_(e)=1.60, λ₁=405 nm and λ₃=785 nm, the following isobtained from Equations (6a) to (6e):$H_{1} = {{h_{ref}\left( {{\lambda = \lambda_{1}},{p = p_{e}}} \right)} = {\frac{\lambda_{1}}{n_{e} - n_{0}} = {0.675\quad{µm}}}}$$H_{i} = {{h_{ref}\left( {{\lambda = \lambda_{3}},{p = p_{e}}} \right)} = {\frac{\lambda_{3}}{n_{e} - n_{0}} = {1.308\quad{µm}}}}$a₀ = 0.516 b₀ = 0 a₁ = 0.516 b₁ = 1 a₂ = 0.937 b₂ = 1${CF}_{2} = {{0 + \frac{1}{1 + \frac{1}{1}}} = \frac{1}{2}}$Thus, CF₂ substantially equals a₀, i.e. the following is met:|CF ₂ −a ₀|=0.016<0.02where 0.02 is a value chosen purely arbitrarily. As a result, it isfound that the limited number #Φ(λ=λ₃, p=p₃) is equal to 2.

In the second case where p₁=p_(e) and p₃=p_(o), and where, e.g.,n_(o)=1.50, n_(e)=1.60, λ₁=405 nm and λ₃=785 nm, the following isobtained from Equations (6a) to (6e):$H_{1} = {{h_{ref}\left( {{\lambda = \lambda_{1}},{p = p_{e}}} \right)} = {\frac{\lambda_{1}}{n_{e} - n_{0}} = {0.675\quad{µm}}}}$$H_{i} = {{h_{ref}\left( {{\lambda = \lambda_{3}},{p = p_{o}}} \right)} = {\frac{\lambda_{3}}{n_{o} - n_{0}} = {1.570\quad{µm}}}}$a₀ = 0.430 b₀ = 0 a₁ = 0.430 b₁ = 2 a₂ = 0.326 b₂ = 3${CF}_{2} = {{0 + \frac{1}{2 + \frac{1}{3}}} = \frac{3}{7}}$Thus, CF₂ substantially equals a₀, i.e. the following is met:|CF ₂ −a ₀=0.001<0.02.As a result, it is found that the limited number #Φ(λ=λ₃, p=p₃) is equalto 7.

Table II shows the limited numbers #Φ(λ=λ₂, p=p₂) and #Φ(λ=λ₃, p=p₃) inrespect of a step height h_(j) equal to h_(rek)(λ=λ₁, p=p_(e)) andh_(ref)(λ=λ₁, p=p_(o)) and in the cases where the polarizations p₂ andp₃ equal p_(e) and/or p_(o). These limited numbers have been calculatedon the theory of Continued Fractions as described above. TABLE II #Φ(λ =λ₂, #Φ(λ = λ₃, p = p₂) p = p₃) p₂ = p_(e) p₂ = p_(o) p₃ = p_(e) p₃ =p_(o) h_(j) = h_(ref)(λ = λ₁, p = p₁) p₁ = p_(e) 8 2 2 7 p₁ = p_(o) 4 83 2

It is noted in Tables I and II that if the polarizations p₁, p₂ and p₃are identical, one of the limited numbers #Φ(λ=λ₂, p=p₂) and #Φ(λ=λ₃,p=p₃) equals 2, i.e. only two different values (zero and π modulo 2π)can be chosen for the corresponding phase changes. This does not allowto design an efficient diffractive part for a non-zeroth order of thecorresponding diffracted beam.

By contrast, it is also noted in Tables I and II that if at least one ofthe polarizations p₁, p₂, p₃ differs from the others, at least threedifferent values can be chosen for Φ(λ=λ₂, p=p₂) and/or Φ(λ=λ₃, p=p₃).The possibility for choosing the phase changes from at least 3 possiblevalues allows to make an efficient diffractive part for each of theradiation beams 15, 15′ and 15″. Furthermore, this advantageously allowsto design the stepped profile with a relatively low number of steps,typically less than 40 steps, since a stepped profile with a high numberof steps (typically, 50 or more steps) is of less practical use.

Furthermore, the values of the phase changes Φ(λ=λ₁, p=p₁), Φ(λ=λ₂,p=p₂) and Φ(λ=λ₃, p=p₃) are chosen such that the diffractive part 24 hasa predetermined transmission efficiency eff_(m) for each diffractionorder m of each of the diffracted radiation beams 15, 15′ and 15″. It isknown, e.g., from said Japanese Patent application JP-A-2001209966 thatthe transmission efficiency “eff_(m)” of the diffractive part 24 for adiffraction order m is given as follows: $\begin{matrix}{{eff}_{m} = {{\frac{1}{T}{\int_{0}^{T}{{A(x)}\exp\left\{ {i\quad{\Phi(x)}} \right\}{\exp\left( {{- i}\frac{2\quad\pi\quad{mx}}{T}} \right)}{\mathbb{d}x}}}}}^{2}} & (7)\end{matrix}$wherein “A(x)” is the transparency amplitude distribution and “T” is thegrating pitch of the diffractive part 24. In the case of a steppedprofile given by Equation (1), Equation (7) can be simplified for thefirst order (m=1): $\begin{matrix}{{eff}_{1} = \left( \frac{P\quad{\sin\left( {\pi/P} \right)}}{\pi} \right)^{2}} & (8)\end{matrix}$It is noted from Equation (8) that the higher the number P is chosen,the higher the efficiency eff₁ is achieved. However, it is also notedthat it may be desirable to use the lowest possible number of steps ineach zone in order to achieve greater manufacturing efficiency for theobjective lens 17.

In the following, and by way of illustration only, it is described howto choose the step heights h_(j) of the stepped profile so as to formeach of the diffracted radiation beams 15, 15′ and 15″ with one maindiffraction order. In the following “m₁”, “m₂” and “m₃” are the maindiffraction orders for the diffracted radiation beams 15, 15′ and 15″,respectively.

Three embodiments of the stepped profile are now described.

The first embodiment of the stepped profile is designed such that m₁=0,m₂=1 and m₃=1. Accordingly, the value Φ(λ=λ₁, p=p₁) substantially equalszero modulo 2π and both the values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) arechosen each among at least three different possible values of phasechanges. It is noted that the three different possible values forΦ(λ=λ₂, p=p₂) may differ from those for Φ(λ=λ₃, p=p₃).

By way of illustration only, in the case where p₁=p_(o), p₂=p_(o) andp₃=p_(e), it is known from Table II that #Φ(λ=λ₃, p=p₂)=8 for p₂=p_(o)and #Φ(λ=λ₃, p=p₃)=3 for p₃=p_(e).

Table III shows the values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) introduced bystep heights that equal qh_(ref)(λ=λ₁, p=p₁) where p₁=p_(o) and “q” isan integer. These values are found from Table I where the values Φ(λ=λ₂,p=p₂) and Φ(λ=λ₃, p=p₃) are known a step height that equalsh_(ref)(λ=λ₁, p=p₁) where p₁=p_(o), i.e. for q=1. TABLE III Φ(λ = λ₂, p= p₂)/2π mod 1 Φ(λ = λ₃, p = p₃)/2π mod 1 q p₂ = p_(o) p₃ = p_(e) 10.623 0.619 2 0.246 0.238 3 0.869 0.857 4 0.492 0.476 5 0.115 0.095 60.738 0.714 7 0.361 0.333 8 0.984 0.952

By way of illustration only, two examples of the first embodiment of thestepped profile are now described where, in the first example, P=4 and,in the second example, P=6.

Regarding the first example (P=4), Table IV shows the ideal value of thephase change “Φ/2π ideal” according to Equation (1), as well as thecorresponding transmission efficiency eff₁ for the first order accordingto Equation (8). Table IV also shows, for a step height qh_(ref)(λ=λ₁,p=p₁), two values of the phase changes “Φ(λ=λ₂, p=p₂)/2π” and “Φ(λ=λ₃,p=p₃)/2π ” which approximate “(D/2π ideal” according to Table II, aswell as the corresponding transmission efficiencies eff₁ for the firstorder (m=1) according to Equation (7), in the case where p₁=p_(o),p₂=p_(o) and p₃=p_(e). TABLE IV Φ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2πmod 1 2π mod 1 Φ/2π ideal q p₂ = p_(o) p₃ = p_(e) j = 1 0.125 5 0.1150.095 j = 2 0.375 7 0.361 0.333 j = 3 0.625 1 0.623 0.619 j = 4 0.875 30.869 0.857 eff₁ 81.1% 81.0% 80.5%

It is noted in Table IV that the transmission efficiency eff₁ has a highvalue (more than 75%) for the first order of both the diffracted beams15′ and 15″, as initially desired. FIG. 6 shows a curve 80 representingthe step height h(x) of the first example (P=4) of the first embodiment(m₁=0, m₂=1, m₃=1) of the diffractive part 24. It is noted in respect ofthe curve 80 that the pattern element is designed such that the relativestep heights h_(j+1)−h_(j) between adjacent steps of said patternelement include a relative step height having an optical pathsubstantially equal to αλ₁, wherein α is an integer and α>1 and λ₁ issaid first wavelength. In other words, such a relative step height ishigher than the reference height h_(ref)(λ=λ₁, p=p₁).

Regarding the second example (P=6), Table V shows the ideal value of thephase change “Φ/2π ideal” according to Equation (1), as well as thecorresponding transmission efficiency eff₁ for the first order accordingto Equation (8). Table V also shows, for a step height qh_(ref)(λ=λ₁,p₁=p_(o)), two values of the phase changes “Φ(λ=λ₂, p=p₂)/2π ” and“Φ(λ=λ₃, p=p₃)/2π” which approximate “Φ/2π ideal” according to TableIII, as well as the corresponding transmission efficiencies eff₁ for thefirst order (m=1) according to Equation (7), in the case where p₁=p_(o),p₂=p_(o) and p₃=p_(e). TABLE V Φ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2πmod 1 2π mod 1 Φ/2π ideal q p₂ = p_(o) p₃ = p_(e) J = 1 0.0833 5 0.1150.095 J = 2 0.2500 2 0.246 0.238 J = 3 0.4167 4 0.492 0.476 J = 4 0.58331 0.623 0.619 J = 5 0.7500 6 0.738 0.714 J = 6 0.9166 8 0.984 0.952 eff₁91.2% 87.4% 87.6%

It is noted in Table V that the transmission efficiency eff₁ has a highvalue (more than 75%) for the first order of both the diffracted beams15′ and 15″, as initially desired. FIG. 7 shows a curve 81 representingthe step height h(x) of the first example (P=4) of the first embodiment(m₁=0, m₂=1, m₃=1) of the diffractive part 24.

In a particular case of the first embodiment of the stepped profile, thevalue Φ(λ=λ₂, p=p₂) is substantially equal to the value Φ(λ=λ₃, p=p₃),where the polarization p₃ different from the polarization p₂, i.e.:Φ(λ=λ₂, p=p₂)=Φ(λ=λ₃, p=p₃)  (9)In the present description, the values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃)are substantially equal where |Φ(Φ=λ₃, p=p₃)−Φ(λ=λ₂, p=p₂)| is less thanor equal to preferably 0.04π, where the value 0.04π is a matter of apurely arbitrary choice.

In the case where p₁=p_(o), p₂=p_(o) and p₃=p_(e) it derives fromEquations (0), (5b), (5c) and (9) that: $\begin{matrix}{\frac{\lambda_{2}}{n_{o} - 1} = \frac{\lambda_{3}}{n_{e} - 1}} & (10)\end{matrix}$It follows from Equation (10) that: $\begin{matrix}{n_{o} = {1 + {\frac{\lambda_{2}}{\lambda_{3}}\left( {n_{e} - 1} \right)}}} & (11)\end{matrix}$Thus, for example, in the case where n_(e)=1.604, λ₂=650 nm and λ₃=785nm, it derives from Equation (11) that n_(o)=1.5. Consequently, thebirefringent material may be chosen where its refractive indices n_(e)and n_(o) substantially equal 1.604 and 1.5, respectively.

In the present description, two refractive indices n_(a) and n_(b) aresubstantially equal where |n_(a)−n_(b)| is equal to or less than,preferably, 0.01 and, more preferably, 0.005, where the values 0.01 and0.005 are a matter of purely arbitrary choice.

Similarly to Table I, Table VI shows the ideal values Φ(λ=λ₂, p=p₂) andΦ(λ=λ₃, p=p₃) where the radiation beams 15′ and 15″ traverse the stepheight h_(j) which equals h_(ref)(λ_(ref)=λ₁, p=p₁) in the case wherep₁=p_(o), p₂=p_(o) and p₃=p_(e). The values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃,p=p₃) have been calculated from Equations (4a), (4b) and (5a) to (5d)with, e.g., n_(o)=1.5, n_(e)=1.604, λ₁=405 nm, λ₂=650 nm and λ₃=785 nm.TABLE VI Φ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2π (modulo 1) 2π(modulo 1) p₂ = p_(o) p₃ = p_(e) h_(j) = h_(ref)(λ_(ref) = λ₁, p = p₁)0.623 0.623 p₁ = p_(o)

Similarly to Table II, Table VII shows the limited numbers #Φ(λ=λ₂,p=p₂) and #Φ(λ=λ₃, p=p₃) in respect of a step height h_(j) equal toh_(ref)(λ=λ₁, p=p₁) in that case where p₁=p_(o), p₂=p_(o) and p₃=p_(e).These limited numbers have been calculated on the theory of ContinuedFractions as described above. TABLE VII #Φ(λ = λ₂, p = p₂) #Φ(λ = λ₃, p= p₃) p₂ = p_(o) p₃ = p_(e) h_(j) = h_(ref)(λ = λ₁, p = p₁) 8 8 p₁ =p_(o)

Similarly to Table III, Table VIII shows the values of phase changeΦ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) introduced by step heights that equalqh_(ref)(λ=λ₁, p=p₁) where “q” is an integer, in the case wherep₁=p_(o), p₂=p_(o) and p₃=p_(e). These values are found from Table VIwhere the values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) where the a step heightthat equals h_(ref)(λ=λ₁, p=p₁), i.e. for q=1. TABLE VIII Φ(λ = λ₂, p =p₂)/ Φ(λ = λ₃, p = p₃)/ 2π (modulo 1) 2π (modulo 1) q p₂ = p_(o) p₃ =p_(e) 1 0.623 0.623 2 0.246 0.246 3 0.869 0.869 4 0.492 0.492 5 0.1150.115 6 0.738 0.738 7 0.361 0.361 8 0.984 0.984

It is noted from Tables VI, VII and VIII that in case of the firstembodiment where p₁=p_(o), p₂=p_(o) and p₃=p_(e) the limited numbers#Φ(λ=λ₂, p=p₂) and #Φ(λ=λ₃, p=p₃) equal 8, i.e. eight different valuescan be chosen for each of Φ(λ=λ₂, p==p₂) and Φ(λ=λ₃, p=p₃).

By way of illustration only, a third example of the first embodiment ofthe stepped profile is now described in the case where P=4.

Similarly to Table IV, Table 1×shows the ideal value of the phase change“Φ/2π ideal” according to Equation (2), as well as the correspondingtransmission efficiency eff₁ for the first order according to Equation(8). Table IX also shows, for a step height qh_(ref)(λ=λ₃, p=p₁), twovalues of the phase changes “Φ(λ=λ₂, p=p₂)/2π” and “Φ(λ=λ₃, p=p₃)/2π”which approximate “Φ/2π ideal” according to Table VIII, as well as thecorresponding transmission efficiencies eff₁ for the first order (m=1)according to Equation (7), in the case where p₁=p_(o), p₂=p_(o) andp₃=p_(e). TABLE IX Φ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2π (modulo 1)2π (modulo 1) Φ/2π ideal q p₂ = p_(o) p₃ = p_(e) j = 1 0.125 5 0.1150.115 j = 2 0.375 7 0.361 0.361 j = 3 0.625 1 0.623 0.623 j = 4 0.875 30.869 0.869 eff₁ 81.1% 81.0% 81.0%

It is noted in Table IX that the transmission efficiency eff₁ has a highvalue (more than 75%) for the first order of both the diffracted beams15′ and 15″, as initially desired. FIG. 8 shows a curve 82 representingthe step height h(x) of the third example (P=4) of the first embodiment(m₁=0, m₂=1, m₃=1) of the diffractive part 24.

The second embodiment of the stepped profile is designed such that m₁=0,m₂=1 and m₃=0. Accordingly, both values Φ(λ=λ₁, p=p₁) and Φ(λ=λ₃, p=p₃)substantially equal zero modulo 2π and the value Φ(λ=λ₂, p=p₂) is chosenamong at least three different phase changes. It is noted that thevalues Φ(λ=λ₁, p=p₁) and Φ(λ=λ₃, p=p₃) (which both substantially equalzero modulo 2π) may however differ from each other.

In a particular case of the second embodiment of the stepped profile,the value Φ(λ=λ₁, p=p₁) is substantially equal to the value Φ(λ=λ₃,p=p₃), where the polarization p₁ differs from the polarization p₃, i.e.:Φ(λ=λ₁ , p=p ₁)=Φ(λ=λ₃ , p=p ₃)  (12)

In the case where p₁=p_(o), p₂=p_(o) and p₃=p_(e) it derives fromEquations (0), (4b), (5c) and (12) that: $\begin{matrix}{\frac{\lambda_{1}}{n_{o} - 1} = \frac{\lambda_{3}}{n_{e} - 1}} & (13)\end{matrix}$It follows from Equation (13) that: $\begin{matrix}{n_{o} = {1 + {\frac{\lambda_{1}}{\lambda_{3}}\left( {n_{e} - 1} \right)}}} & (14)\end{matrix}$Thus, for example, in the case where n_(e)=1.722, λ₁=405 nm and λ₃=650nm, it derives from Equation (14) that n_(o)=1.45. Consequently, thebirefringent material may be chosen where its refractive indices n_(e)and n_(o) substantially equal 1.722 and 1.45, respectively.

Similarly to Table I, Table X shows the ideal values Φ(λ=λ₂, p=p₂) andΦ(λ=λ₃, p=p₃) where the radiation beams 15′ and 15″ traverse the stepheight h_(j) which equals h_(ref)(λ_(ref)=λ₁, p=p₁) in the case wherep₁=p_(o), p₂=p_(e) and p₃=p_(e). The values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃,p=p₃) have been calculated from Equations (4a), (4b) and (5a) to (5d)with, e.g., n_(o)=1.45, n_(e)=1.722, λ₁=405 nm, λ₂=785 nm and λ₃=650 nm.TABLE X Φ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2π (modulo 1) 2π(modulo 1) p₂ = p_(e) p₃ = p_(e) h_(j) = h_(ref)(λ_(ref) = λ₁, p = p₁)0.828 0 p₁ = p_(o)

Similarly to Table II, Table XI shows the limited numbers #Φ(λ=λ₂, p=p₂)and #Φ(λ=λ₃, p=p₃) in respect of a step height h_(j) equal toh_(ref)(λ=λ₁, p=p₁) in that case where p₁=p_(o), p₂=p_(e) and p₃=p_(e).These limited numbers have been calculated on the theory of ContinuedFractions as described above. TABLE XI #Φ(λ = λ₂, p = p₂) #Φ(λ = λ₃, p =p₃) p₂ = p_(e) p₂ = p_(e) h_(j) = h_(ref)(λ = λ₁, p = p₁) 6 1 p₁ = p_(o)

Similarly to Table m, Table XII shows the values of phase change Φ(λ=λ₂,p=p₂) and Φ(λ=λ₃, p=p₃) introduced by step heights that equalqh_(ref)(λ=λ₁, p=p_(o)) where “q” is an integer. These values are foundfrom Table X where the values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) are knowna step height that equals h_(ref)(λ=λ₁, p=p₁), i.e. for q=1. TABLE XIIΦ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2π (modulo 1) 2π (modulo 1) q p₂ =p_(e) p₃ = p_(e) 1 0.828 0.0 2 0.656 0.0 3 0.484 0.0 4 0.312 0.0 5 0.1400.0 6 0.968 0.0 7 0.796 0.0

It is noted from Tables X, XI and XII that in case of the secondembodiment where p₁=p_(o), p₂=p_(e) and p₃=p_(e) the limited number#Φ(λ=λ₂, p=p₂) equal 6, i.e. six different values can be chosen forΦ(λ=λ₂, p=p₂).

By way of illustration only, an example of the second embodiment of thestepped profile is now described where P=4.

Similarly to Table IV, Table XIII shows the ideal value of the phasechange “Φ/2π ideal” according to Equation (1), as well as thecorresponding transmission efficiency eff₁ for the first order accordingto Equation (8). Table XII also shows, for a step height qh_(ref)(λ=λ₁,p=p₁), two values of the phase changes “Φ(λ=λ₂, p=p₂)/2π” and “Φ(λ=λ₂,p=p₂)/2π” which approximate “Φ/2π ideal” according to Table XII, as wellas the corresponding transmission efficiencies eff₁ for the first order(m=1) according to Equation (7), in the case where p₁=p_(o), p₂=p_(e)and p₃=p_(e). TABLE XIII Φ/2π ideal q Φ(λ = λ₂, p = p₂)/2π (modulo 1) j= 1 0.125 5 0.140 j = 2 0.375 4 0.312 j = 3 0.625 2 0.656 j = 4 0.875 10.828 eff₁ 81.1% 76.1%

It is noted in Table XIII that the transmission efficiency eff₁ has ahigh value (more than 75%) for the first order of the diffracted beams15′, as initially desired. FIG. 9 shows a curve 83 representing the stepheight h(x) of the third example (P=4) of the second embodiment (m₁=0,m₂=1, m₃=0) of the diffractive part 24.

The third embodiment of the stepped profile is designed such that m₁=0,m₂=1 and m₃=0. Accordingly, the value Φ(λ=λ₁, p=p₁) substantially equalszero modulo 2π, the value Φ(λ=₂, p=p₂) is chosen among at least threedifferent phase changes, and the value Φ(λ=λ₃, p=p₃) substantiallyequals π modulo 2π.

Similarly to Table I, Table XIV shows the ideal values Φ(λ=λ₂, p=p₂) andΦ(λ=λ₃, p=p₃) where the radiation beams 15′ and 15″ traverse the stepheight h_(j) which equals h_(ref)(λ_(ref)=λ₁, p=p₁) in the case wherep₁=p_(e), p₂=p_(o) and p₃=p_(o). The values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃,p=p₃) have been calculated from Equations (4a), (4b) and (5a) to (5d)with, e.g., n_(o)=1.50, n_(e)=1.60, λ₁=405 nm, λ₂=785 nm and λ₃=650 nm.TABLE XIV Φ(λ = λ₂, p = p₂)/ Φ(λ = λ₃, p = p₃)/ 2π (modulo 1) 2π(modulo 1) p₂ = p_(o) p₃ = p_(o) h_(j) = h_(ref)(λ_(ref) = λ₁, p = p₁)0.430 0.519 p₁ = p_(e)

Similarly to Table II, Table XV shows the limited numbers #Φ(λ=λ₂, p=p₂)and #Φ(λ=λ₃, p=p₃) in respect of a step height h_(j) equal toh_(ref)(λ=λ₁, p=p₁) in that case where p₁=p_(e), p₂=p_(o) and p₃=p_(o).These limited numbers have been calculated on the theory of ContinuedFractions as described above. TABLE XV #Φ(λ = λ₂, p = p₂) #Φ(λ = λ₃, p =p₃) p₂ = p_(o) p₃ = p_(o) h_(j) = h_(ref)(λ = λ₁, p = p₁) 7 2 p₁ = p_(e)

Similarly to Table III, Table XVI shows the values of phase changeΦ(λ=λ₂, p=p₂) and Φ(λ=λ₃, p=p₃) introduced by step heights that equalqh_(ref)(λ=λ₁, p=p₁) where p₁=p_(e) and “q” is an integer. These valuesare found from Table XIV where the values Φ(λ=λ₂, p=p₂) and Φ(λ=λ₃,p=p₃) are known a step height that equals h_(ref)(λ=λ₁, p=p₁), i.e. forq=1. TABLE XVI Φ(λ = λ₂, p = p₂)/2π Φ(λ = λ₃, p = p₃)/2π (modulo 1) m(modulo 1) p₂ = p_(o) p₃ = p_(o) 1 0.430 0.519 2 0.860 0.038 3 0.2900.557 4 0.720 0.076 5 0.150 0.595 6 0.580 0.114 7 0.010 0.633 8 0.4400.152 9 0.870 0.671 10 0.300 0.190 11 0.730 0.709 12 0.160 0.228

It is noted from Tables XIV, XV and XVI that in case of the thirdembodiment where p₁=p_(e), p₂=p_(o) and p₃=p_(o) the limited number#Φ(λ=λ₃, p=p₃) equal 2, i.e. two different values can be chosen forΦ(λ=λ₃, p=p₃), and the limited number #Φ(λ=λ₂, p=p₂) equal 7, i.e. sevendifferent values can be chosen for Φ(λ=_(λ2), p=p₂).

By way of illustration only, an example of the third embodiment of thestepped profile is now described where P=4.

Similarly to Table IV, Table XVII shows for a step height qh_(ref)(λ=λ₁,p=p₁), two values of the phase changes “Φ(λ=λ₂, p=p₂)/2π ” and “Φ(λ=λ₃,p=p₃)/2π” which approximate “Φ/2π ideal” according to Table XVI, as wellas the corresponding transmission efficiencies eff₁ for the first order(m=1) according to Equation (7), in the case where p₁=p_(e), p₂=p_(o)and p₃=p_(o). TABLE XVII Φ(λ = λ₂, p = p₂)/2π Φ(λ = λ₃, p = p₃)/2π(modulo 1) m (modulo 1) p₂ = p_(o) p₃ = p_(o) j = 1 5 0.150 0.595 j = 21 0.430 0.519 j = 3 11 0.730 0.709 j = 4 9 0.870 0.671 eff₀ 1.8% 80.6%eff₁ 75.9% 4.5%

It is noted in Table XVII that the four phase steps introduce phasechanges for the wavelength λ₃ having substantially the same value.

It is also noted in Table XVII that the transmission efficiency eff₁ hasa low value (less than 5%) for the first order of the diffracted beams15″ and a high value (more than 75%) for the first order of thediffracted beams 15′, as initially desired. FIG. 10 shows a curve 84representing the step height h(x) of that example (P=4) of the thirdembodiment (m₁=0, m₂=1, m₃=0) of the diffractive part 24.

Once the stepped profile of each pattern element has been designed asdescribed above in respect of any of the first, second and thirdembodiments, the pattern of the pattern elements of the diffractive part24 is designed so that the combination of the objective lens system 8and the diffractive part 24 has a first focusing characteristic for theradiation beam 4, a second focusing characteristic for the radiationbeam 4′, and a third focusing characteristic for the radiation beam 4″.

In this embodiment, the pattern is designed so that the combination ofthe objective lens 17 and the diffractive part 24 corrects sphericalaberration caused by the difference between the information layer depths27 and 27′ (due to the difference in thickness of the transparent layers5 and 5′) and between the information layer depths 27 and 27″ (due tothe difference in thickness of the transparent layers 5 and 5″).

More specifically, the pattern of the diffractive part 24 is designed asa circular grating structure having a pattern of coaxially ring-shapedpattern elements with gradually increasing width towards the center ofthe objective lens 17 (as shown in FIG. 3). As a result, the combinationof the diffractive part 24 and the objective lens 17 focuses theradiation beam 15 in the information layer 2 having the firstinformation layer depth 27 and no spherical aberration is generated whenthe radiation beam 15 emerges from the objective lens 17 (the firstfocusing characteristic).

Furthermore, the pattern of the diffractive part 24 is designed togenerate, in combination with the objective lens 17, an amount ofspherical aberration which is proportional to m₁λ₁−m₂λ₂ (i.e. in thisembodiment −λ₂ since m₁=0 and m₂=1). As a result, the combination of thediffractive part 24 and the objective lens 17 focuses the radiation beam15′ in the information layer 2′ having the information layer depth 27′and spherical aberration generated due the difference in thickness ofthe transparent layer is compensated (the second focusingcharacteristic).

Similarly, the pattern of the diffractive part 24 is designed togenerate, in combination with the objective lens 17, another amount ofspherical aberration which is proportional to m₁λ₁−m₃λ₃ (i.e. in thisembodiment −λ₃ since m₁=0 and m₃=1). As a result, the combination of thediffractive part 24 and the objective lens 17 focuses the radiation beam15″ in the information layer 2″ having the information layer depth 27′and spherical aberration generated due the difference in thickness ofthe transparent layer is compensated (the third focusingcharacteristic). It is noted in this embodiment that both the second andthird focussing characteristics differ from the first focussingcharacteristic, as shown in FIG. 2.

Whilst in the above described embodiment an optical scanning devicecompatible with a CD-format disc, a “Red-DVD”-format disc and a“Blue-DVD”-format disc is described, it is to be appreciated that thescanning device according to the invention can be alternatively used forany other types of optical record carriers to be scanned.

An alternative of the stepped profiles described above may be designedfor forming diffracted radiation beams for diffraction order other thanthe zeroth order and the first order, or a combination of orders with amain selected order and at least another selected order.

In other alternatives of the stepped profiles described above, thewavelength λ₂ or λ₃ is chosen as the design wavelength. Table XVIIIshows the values of the reference height h_(ref)(λ, p) in the case wherethe wavelength λ equals λ₂ or λ₃ and the polarization p equals p_(o) orp_(e) and where, e.g., n_(o)=1.5, n_(e)=1.6, λ₂₌₆₅₀ nm and λ₃=785 nm.TABLE XVIII h_(ref)(λ, p) λ = λ₂ λ = λ₃ p = p_(e) 1.083 μm 1.308 μm p =p_(o) 1.300 μm 1.570 μm

As an alternative to the pattern of the pattern elements described abovefor correcting spherical aberration, the pattern is designed, e.g., forcorrecting spherochromatism and chromatic aberration or for carrying outthe three-spot push pull method by forming the main spot from the firstdiffracted beam with a transmission efficiency equal to, e.g., 80% andthe two satellite spots from the second and third diffracted beams witha transmission efficiency equal to, e.g., 10% for each beam.

An alternative to the diffractive part arranged on the entrance face ofthe objective lens may be a grating of any shape like a plane grating,since the stepped-profile of each pattern element is determinative forthe transmission efficiency in respect of each order, irrespective ofthe shape of the grating.

As an alternative to the optical scanning device described withwavelengths of 785 nm, 660 nm and 405 nm are used, it is to beappreciated that radiation beams of any other combinations ofwavelengths suitable for scanning optical record carriers may be used.

As another alternative to the optical scanning device described with theabove values of numerical apertures in respect of the first, second andthird diffracted beams, it is to be appreciated that radiation beams ofany other combinations of numerical apertures suitable for scanningoptical record carriers may be used.

1. An optical scanning device for scanning a first information layer by means of a first radiation beam having a first wavelength and a first polarization, a second information layer by means of a second radiation beam having a second wavelength and a second polarization, and a third information layer by means of a third radiation beam having a third wavelength and a third polarization, wherein said first, second and third wavelengths substantially differ from each other and at least one of said first, second and third polarizations differs from the others, the device comprising: a radiation source for supplying said first, second and third radiation beams consecutively or simultaneously, an objective lens system for converging said first, second and third radiation beams on the positions of said first, second and third information layers, respectively, and a diffractive part arranged in the optical path of said first, second and third radiation beams, the part including a pattern of pattern elements which have substantially one stepped profile for forming a first diffracted radiation beam, a second diffracted radiation beam and a third diffracted radiation beam from said first, second and third radiation beams, respectively, the part comprising birefringent material sensitive to said first, second and third polarizations, characterized in that said stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two different multiples of 2π for said first wavelength and at least two substantially different phase changes modulo 2π for said second wavelength.
 2. An optical scanning device according to claim 1, wherein said stepped profile is further designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two substantially different phase changes modulo 2π for said third wavelength.
 3. An optical scanning device according to claim 2, wherein said stepped profile is further designed such that the heights of the steps of a pattern element introduce substantially identical phase changes for both said second and third wavelengths, wherein said third polarization differs from said second polarization.
 4. An optical scanning device according to claim 3, wherein the extraordinary refractive index of said birefringent material substantially equals ${1 + {\frac{\lambda_{c}}{\lambda_{b}}\left( {n_{o} - 1} \right)}},$ where “n_(o)” is the ordinary refractive index of said birefringent material and “λ_(b)” and “λ_(c)” are either said second and third wavelengths, respectively, or said third and second wavelengths, respectively.
 5. An optical scanning device according to claim 1, wherein said stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two different multiples of 2π for said third wavelength.
 6. An optical scanning device according to claim 5, wherein said stepped profile is further designed such that the heights of the steps of a pattern element introduce substantially identical phase changes for both said first and third wavelengths, wherein said third polarization differs from said first polarization.
 7. An optical scanning device according to claim 6, wherein the extraordinary refractive index of said birefringent material substantially equals ${1 + {\frac{\lambda_{c}}{\lambda_{b}}\left( {n_{o} - 1} \right)}},$ where “n_(o)” is the ordinary refractive index of said birefringent material and “λ_(b)” and “λ_(c)” are either said first and third wavelengths, respectively, or said third and first wavelengths, respectively.
 8. An optical scanning device according to claim 1, wherein said stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two different odd multiples of π for said third wavelength.
 9. An optical scanning device according to claim 8, wherein said stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two of an odd number of substantially different phase changes for said second wavelength.
 10. An optical scanning device according to claim 1, wherein said pattern element is designed such that the relative step heights between adjacent steps of said pattern element include a relative step height having an optical path substantially equal to αλ₁, wherein α is an integer and α>1 and λ₁ is said first wavelength.
 11. An optical scanning device according to claim 1, wherein the shape of said diffractive part is generally circular and the steps of said pattern element are generally annular.
 12. An optical scanning device according to claim 1, wherein said diffractive part is formed on a face of a lens of the objective lens system.
 13. An optical scanning device to claim 1, wherein said diffractive part is formed on an optical plate provided between said radiation source and said objective lens system.
 14. An optical scanning device according to claim 13, wherein said optical plate comprises a quarter wavelength plate or a beam splitter.
 15. A diffractive part for use in an optical device for scanning a first information layer by means of a first radiation beam having a first wavelength and a first polarization, a second information layer by means of a second radiation beam having a second wavelength and a second polarization, and a third information layer by means of a third radiation beam having a third wavelength and a third polarization, wherein said first, second and third wavelengths substantially differ from each other and at least one of said first, second and third polarizations differs from the others, the diffractive part: being arranged in the optical path of said first, second and third radiation beams, including a pattern of pattern elements which have substantially one stepped profile for forming a first diffracted radiation beam, a second diffracted radiation beam and a third diffracted radiation beam from said first, second and third radiation beams, respectively, and comprising birefringent material sensitive to said first, second and third polarizations, characterized in that said stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two different multiples of 2π for said first wavelength, at least two substantially different phase changes modulo 2π for said second wavelength and, for said third wavelength, one of the following: at least two substantially different phase changes modulo 2π, at least two different multiples of 2π, or at least two different odd multiples of π.
 16. A lens for use in an optical device for scanning a first information layer by means of a first radiation beam having a first wavelength and a first polarization, a second information layer by means of a second radiation beam having a second wavelength and a second polarization, and a third information layer by means of a third radiation beam having a third wavelength and a third polarization, wherein said first, second and third wavelengths substantially differ from each other and at least one of said first, second and third polarizations differs from the others, the lens being provided with a diffractive part according to claim
 15. 